libstdc++: Use __is_single_threaded() in performance tests

With recent glibc releases the __gthread_active_p() function is always
true, so we always append "-thread" onto performance benchmark names.

Use the __gnu_cxx::__is_single_threaded() function instead.

libstdc++-v3/ChangeLog:

	* testsuite/util/testsuite_performance.h: Use
	__gnu_cxx::__is_single_threaded instead of __gthread_active_p().